Conventionally, in image forming apparatuses that can perform color printing such as a copying machine, a printer, and a multi-functional peripheral (MFP), an electrophotographic system called tandem type is widely used.
In an image forming apparatus of a tandem electrophotographic system, four photoconductive drums corresponding to respective colors of yellow (Y), magenta (M), cyan (C), and black (K) are arrayed along an intermediate transfer belt. Images of the respective colors of Y, M, C, and K are transferred from the respective photoconductive drums onto an intermediate transfer belt to be superimposed one on top of another. A full-color image is formed on the intermediate transfer belt. This full-color image is further transferred onto a sheet and a full-color image is formed on the sheet.
The transfer of the images from the respective photoconductive drums onto the intermediate transfer belt is performed by using four transfer rollers provided for the respective colors of Y, M, C, and K. The four transfer rollers are arrayed in positions opposed to the photoconductive drums for the respective colors across the intermediate transfer belt.
In general, an image forming apparatus that can perform color printing has two printing modes, i.e., a full-color printing mode for forming a full-color image and a monochrome printing mode for forming a monochrome (black and white) image.
As disclosed in JP-A2004-163795 and so on, in the full-color printing mode, the four transfer rollers are shifted to positions where the transfer rollers are brought into press contact with the photoconductive drums for the corresponding colors via the intermediate transfer belt (hereinafter, press-contact position). On the other hand, in the monochrome printing mode, the three transfer rollers for Y, M, and C are shifted to positions where the transfer rollers are separated from the photoconductive drums of the corresponding colors and the intermediate transfer belt (hereinafter, separated position). As a result, components unnecessary in the monochrome printing mode relatively frequently used, i.e., the respective transfer rollers and the respective photoconductive drums for Y, M, and C do not physically come into contact with the intermediate transfer belt, and thus it is possible to extend the life of these components and improve reliability thereof.
The image forming apparatus can switch the full-color printing mode and the monochrome printing mode according to setting by a user. The image forming apparatus can also automatically switch the full-color printing mode and the monochrome printing mode by distinguishing whether an original is a color original or a monochrome original. In the case of the automatic switching, the full-color printing mode and the monochrome printing mode are likely to be frequently switched depending on a type of an original. Therefore, it is necessary to shift the three transfer rollers for Y, M, and C between the press-contact position and the separating position in a short time.
On the other hand, if a driving force is set too large in order to shift the transfer rollers between the press-contact position and the separating position in a short time, power necessary for driving increases. Moreover, impact involved in shifting and stopping of the transfer rollers increases, which causes noise and wear of components.